Abstract
The clearance flow around the pump-turbine runner has significant influences on unit vibrations, which may cause accidents in transient processes. The dynamic hydraulic forces and flow patterns in the clearance flow channel (CFC) of a low specific-speed pump-turbine were analyzed based on 3D CFD simulations during the runaway oscillating process. It is shown that the axial force of the runner periodically fluctuates with large amplitudes, and its components in CFC and the main flow channel (MFC) demonstrate a similar significance level. The CFC component was formulated as a function of the clearance inlet pressure and rotational speed, while the MFC component as a function of the momentum changing rate and the runner outlet pressure force. The fluctuation of runner radial force is mainly caused by the flow evolution in MFC, however, the flow in CFC aggravates it. The pressure in CFC shows a few pulsating signals from MFC, and the radial pressure drop in CFC is proportional to the square of both radius and rotational speed. In CFC, strong rotating shear flow containing a velocity core region in the circumferential direction is formed, and rotational speed is the dominant factor.
Highlights
As the main way to store energy on large scale, the pumped-storage power generally undertakes functions such as peak load regulation, valley load regulation, frequency modulation, phase modulation and spinning reserve in the power grid, to match with thermal, nuclear, wind and solar powers
The runner forces can be classified into two parts: the force in the main flow channel (MFC) and the force in the clearance flow channel (CFC), which is the small gaps between the rotating and stationary parts of the pump-turbine
It was recognized that the pressure, distribution and evolution in CFC have significant influences on the total runner forces
Summary
As the main way to store energy on large scale, the pumped-storage power generally undertakes functions such as peak load regulation, valley load regulation, frequency modulation, phase modulation and spinning reserve in the power grid, to match with thermal, nuclear, wind and solar powers. Xia et al [9,10] numerically studied pressure pulsations and runner forces of a model pump-turbine during the runaway transient process and found that the reverse flow around the runner inlet could cause high-frequency axial force fluctuations. The dynamic hydraulic forces and flow patterns in CFC of a low specific-speed pump-turbine were analyzed based on three-dimensional (3D) CFD simulations of the runaway oscillating process. Where p is the pressure; ρ is the density of water; ω0 is the rated rotational speed; u1 is the circumferential velocity at R1 in the rated operating condition; v is the flow velocity; R1 is the radius of the runner inlet, equivalent to the maximum radius of CFC; r is the radius in the clearance area; Rm is the radius at the inlet of the sealing ring of CHC (Figure 1c).
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